Isn't this getting a little complicated and costly?? And taking up a lot of real estate? For something that might not work? Use a DIY tank with a plate hx and get on with life. This is a fun exersize, but even I, a chronic tinkerer, see that creating a vessel to hold wet, moist, or saturated sand is a lot like a water tank, but only a lot bigger and more difficult to move heat into or remove heat from. A solar system inputs much lower amounts of energy than a wood boiler. Follow the btu's!!